Low-cycle fatigue of nickel superalloy single crystals at elevated temperatures

Single-crystal samples of nickel superalloys containing rhenium or rhenium plus ruthenium are subjected to low-cycle fatigue (LCF) tests under rigid cycle conditions at temperatures of 850 and 1050°C. It is found that the single crystals of the alloy containing rhenium with ruthenium have higher LCF resistance at 10⁴ cycles as compared to the alloy containing only rhenium. At a test temperature of 850°C, volume stress concentrators in the form of pores or their clusters represent fatigue crack nucleation zones; at 1050°C, surface corrosion cracks are the main fracture zones. The fatigue microcrack growth rate is anisotropic: it is higher in the [001] direction and lower in the [011] direction.     

Induced creep and creep/fatigue of a nickel-base superalloy at ambient temperatures

The stress controlled fatigue of Nimonic*115, a typical γ’-strengthened nickel-base superalloy, was studied at ambient temperature, using a trapezoidal wave form at 1 Hz, with stresses chosen to produce failure in the lO⁴ to lO⁴ cycle range. In tests with maximum stress greater than the proportional limit, most of the fatigue damage occurs within the first few test cycles. Much of this strain is accumulated under static load and is therefore identified as creep strain. Transmission electron microscopy shows that these creep strains occur in slip bands which disrupt the ordered γ’ precipitates. Strain is found to follow a logarithmic time dependence, which suggests a low activation energy mechanism.     

Low cycle fatigue behavior of polycrystalline Ni3Al alloys at ambient and elevated temperatures

The low cycle fatigue (LCF) resistance of polycrystalline Ni₃Al has been evaluated at ambient, intermediate (300 °C), and elevated (600 °C) temperatures using strain rates of 10⁻²/s and 10⁻⁴/s. Testing was conducted on a binary and a Cr-containing alloy of similar stoichiometry and B content (hypostoichiometric, 200 wppm B). Test results were combined with electron microscope investigations in order to evaluate microstructural changes during LCF. At ambient and intermediate temperatures, the cyclic constitutive response of both alloys was similar, and the LCF behavior was virtually rate independent. Under these conditions, the alloys rapidly hardened and then gradually softened for the remainder of the life. Initial hardening resulted from the accumulation of dislocation debris within the deformed microstructure, whereas softening was related to localized disordering. For these experimental conditions, crack initiation resulted within persistent slip bands (PSBs). At the elevated temperature, diffusion-assisted deformation resulted in a rate-dependent constitutive response and crack-initiation characteristics. At the high strain rate (10⁻²/s), continuous cyclic hardening resulted from the accumulation of dislocation debris. At the low strain rate (10⁻⁴/s), the diffusion of dislocation debris to grain boundaries resulted in cyclic softening. The elevated temperature LCF resistance was determined by the effect of the constitutive response on the driving force for environmental embrittlement. Chromium additions were observed to enhance LCF performance only under conditions where crack initiation was environmentally driven. Formerly Postdoctoral Research Fellow, School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA     

Estimation of ionic activities in chloride systems at ambient and elevated temperatures

The study of non-oxidative dissolution of sulfide minerals in HCl solutions requires a thorough knowledge of the thermodynamic properties of the leaching medium. In particular, estimates of single-ion activities are necessary.This paper shows that a consistent set of data may be derived from reported experimental results for the pure components at 25°C. Estimated ion activities may be obtained from ionic hydration theory, extended to allow for a decreasing hydration number at low water activities. A relationship is derived for ionic activities in a mixture of a univalent and a bivalent salt with a common, unhydrated anion. Selected data are presented for HCl, NaCl, and MgCl₂, as well as for mixtures of HCl with the two salts, at 25–100°C.     

一种单晶高温合金不同温度的高周疲劳性能

研究了一种单晶高温合金700 ℃和800 ℃的高周疲劳性能，采用扫描电镜和透射电镜分析了断口和断裂机制.结果表明，随着温度升高，合金的疲劳强度系数降低，Basquin系数增加，高周疲劳极限降低.合金700 ℃与800 ℃具有相同的高周疲劳断口，都有几个{111}面平面组成，为类解理断裂机制.疲劳断口由裂纹源区、扩展区和瞬断区3部分组成.裂纹起源于试样的表面或亚表面，并沿{111}面扩展.扩展区可见河流状花样、滑移带、疲劳弧线和疲劳条带特征.瞬断区可见解理台阶和撕裂棱.断裂后γ′相仍保持立方形状，位错不均匀分布在γ基体通道中.      

Fatigue crack growth through partially stabilized zirconia at ambient and elevated temperatures

Fatigue crack growth through magnesia stabilized zirconia at 20, 450 and 650°C has been observed dynamically in a high temperature loading stage for the scanning electron microscope. Crack tip micromechanics parameters were measured using the stereoimaging technique. Fatigue crack growth at ambient temperature was found to be very similar to crack growth through metallic alloys. With increasing temperature, the stress intensity levels in which stable fatigue crack growth could be sustained were found to narrow significantly, until fatigue is expected to not be a valid mechanism of crack growth above about 750°C. Measured crack tip parameters were used to derive the low-cycle fatigue and the stress-cycles to failure characteristics. The latter agreed with measured SN curves. Deformation within the plastic zone was shown to account for the measured value of fracture toughness. The mechanisms of crack growth are discussed.     

cyclic deformation of Ni3(Al,Nb) single crystals at ambient and elevated temperatures

In order to study the cyclic deformation of an alloy crystal and the asymmetry in the flow stress of the γ′ phase, Ni₃(Al, Nb) crystals have been cycled in strain control at room temperature, 400 and 700°C. The orientation of the crystals has been a major variable studied. At all temperatures, cyclic hardening has been found considerable and at the two lower temperatures, the asymmetry has been found to follow the predictions of the model by Paidar et al. [Acta metall. 32, 435 (1984)] which is based on the increase of the dislocation friction stress by thermally activated cross slip onto [001] planes. Moreover, TEM observations of the dislocation structures are consistent with the model. At 700°C, a dominance of the tensile stress in the asymmetry was observed rather surprisingly. The model of Paidar et al. does not apply at this high temperature because dislocation climb and cube slip occurs, and no asymmetry is expected. Cracking and life behavior are also reported.     

Low cycle fatigue behavior of Ni-Nr lamellar eutectic composites at elevated temperatures

Low cycle fatigue properties of unidirectionally solidified lamellar eutectic Ni-51 Cr alloy were determined and compared with those of the cast microstructure in the temperature range of 300° to 760°C. Both materials exhibited an initial cyclic strain hardening followed by saturation over most of the temperature range. The rate and the amount of cyclic work-hardening decreased with temperature above 600°C. Rapid softening due to macro-crack propagation occurred at later stages of the fatigue process, which occupied an increasing portion of the fatigue life in the lamellar material as the strain amplitude was decreased. At Δ∈_T = 0.0190, the lamellar material exhibited longer fatigue life over the entire temperature range which has been related to the ability of Cr-rich lamellae to deflect fatigue cracks. At 625°C, the fatigue life (Nf) of both materials was related to the plastic strain range ( Δ∈_P) through the relationship (Δ∈_P/2 =K(2N_f)^c wherec andK are -0.39 and 0.068 for the lamellar, and -0.45 and 0.074 for the cast structure, respectively. At this temperature with decreasing strain amplitude lamellar material became more resistant to fatigue than as-cast structure, which has been related to the more efficient deflection of fatigue cracks by Cr-rich lamellae at lower strain amplitudes . Formerly with the Dept. of Metallurgical and Materials Engineering, University of Pittsburgh, Pittsburgh, Pa. Formerly Visiting Scientist, Department of Metallurgical and Materials Engineering, University of Pittsburgh Formerly Professor and Chairman, Department of Metallurgical and Materials Engineering, University of Pittsburgh     

The dislocation structures of Ni3(Al,Nb) single crystals fatigued at ambient and elevated temperatures

In order to support companion studies of the cyclic stress-strain response of Ni₃(Al,Nb) crystals, the dislocation structures of crystals cycled at ambient temperature, 400 and 700°C, have been studied by TEM. Unit or dipolar screw dislocations were observed in high density for the two lower temperatures. These results support the theory of the cross-slip-induced friction stress on the dislocations, which explains the anomalous increase of the flow stress with increase of temperature as well as the tension-compression asymmetry notable in the cyclic results. For the crystals tested at 700°C, the character of the dislocation structure was found to change and to include a higher density of edge dislocations, consistent with saturation of the temperature effect on the flow stress and the onset of cube slip.     

Fracture and fatigue behavior of sintered steel at elevated temperatures: Part I. Fracture toughness

This study investigates fracture resistance of a sintered steel in the temperature range from 25 °C to 300 °C. The temperature-dependent fracture resistance is experimentally determined by fracture toughness tests. The fracture toughness, K _IC , decreases from 28.8 at room temperature to 23 MPa√m at 300 °C. The finite element analysis shows an insight of the rationale of using K _IC as the parameter to characterize the fracture resistance of porous sintered steel in which the stress intensity (K) field has been severely distorted at the porous crack tip. The analysis indicates that crack onset of sintered steel is controlled by a critical stress mechanism.     

Mechanisms of damage and fracture in high-temperature,low-cycle fatigue of a cast nickel-based superalloy

In cast Udimet 500 subjected to high-temperature, low-cycle fatigue, localized oxidation at grain boundaries plays an important role in crack nucleation and propagation. Evidence is presented of a surface ridging and pronounced grain boundary penetration due to oxidation, a denudedγ′ zone adjacent to the oxide, and cracking of the oxide. The ridging is selective, and is presumed to occur on those boundaries where high stress exists. The phenomenon is viewed as analogous to stress-corrosion cracking.     

Crack propagation of steels during low cycle thermal-mechanical and isothermal fatigue at elevated temperatures

The surface and through crack propagation of three high temperature steels in low cycle thermal-mechanical and isothermal fatigue at elevated temperatures was investigated. The rate of crack propagation obtained was correlated with the range of cyclicJ-integral, ΔJf. It was found that there is a linear relationship on alog-log plot regardless of materials, test conditions, and crack configurations. Furthermore, fatigue life predicted by integrating the equation of crack propagation was compared with experimental results.     

Fracture and fatigue behavior of sintered steel at elevated temperatures: Part II. Fatigue crack propagation

Fatigue cracking resistance of sintered steel as a function of temperature is characterized by crack growth rate vs the stress intensity range, ΔK. The stress ratio effects on fatigue crack propagation (FCP) are investigated from room temperature to 300 °C. The crack closure effects on FCP are evaluated by both theoretical and experimental approaches. We found that the crack closure cannot be fully responsible for the observed increase of fatigue resistance with low stress ratio. Experimental results support that both K _max and ΔK control near-threshold crack growth. Fatigue crack resistance at high ΔK regime decreases with temperature. The apparent increase of fatigue resistance at the near-threshold regime at elevated temperatures might be attributed to microcrack toughening.     

The deformation and fracture of Ti3Al at elevated temperatures

The tensile properties of the intermetallic compound Ti₃Al have been determined in air at several temperatures within the range of 25 to 900 °C. The dislocation structures produced by the various testing conditions were studied in the electron microscope and the fracture modes were studied in the scanning electron microscope. These microstructural observations were correlated with the mechanical properties. The results indicate that Ti₃Al has only limited ductility even at 900 °C. The apparent ductile-brittle transition which occurs above 600 °C is due to increasing amounts of intergranular cracking. Some increase in ductility above 600 °C is due to the onset of dislocation cross slipping. The fracture mode up to 600 °C is entirely cleavage. Above 600 °C the fracture shows increasing evidence of plasticity; however, cleavage remains the main fracture mode up to 900 °C. Formerly with the Materials Laboratory Formerly in the Processing and High Temperature Materials Branch     

Lattice misfits in four binary Ni-Base γ/γ1 alloys at ambient and elevated temperatures

High-temperature X-ray diffractometry was used to determine thein situlattice parameters,a _γ anda _γ′, and lattice misfits, δ = (a _γ′, -a _γ)/a _γ, of the matrix (γ) and dispersed γ′-type (Ni₃X) phases in polycrystalline binary Ni-Al, Ni-Ga, Ni-Ge, and Ni-Si alloys as functions of temperature, up to about 680 °C. Concentrated alloys containing large volume fractions of theγ′ phase (∼0.40 to 0.50) were aged at 700 °C to produce large, elastically unconstrained precipitates. The room-temperature misfits are 0.00474 (Ni-Al), 0.01005 (Ni-Ga), 0.00626 (Ni-Ge), and -0.00226 (Ni-Si), with an estimated error of ± 4 pct. The absolute values of the lattice constants of theγ andγ′ phases, at compositions corresponding to thermodynamic equilibrium at about 700 °C, are in excellent agreement with data from the literature, with the exception of Ni₃Ga, the lattice constant of which is much larger than expected. In Ni-Ge alloys, δ decreases to 0.00612 at 679 °C, and in Ni-Ga alloys, the decrease is to 0.0097. In Ni-Si and Ni-Al alloys, δ exhibits a stronger temperature dependence, changing to-0.00285 at 683 °C (Ni-Si) and to 0.00424 at 680 °C (Ni-Al). Since the times required to complete the high-temperature X-ray diffraction (XRD) scans were relatively short (2.5 hours at most), we believe that the changes in δ observed are attributable to differences between the thermal expansion coefficients of theγ andγ′ phases, because the compositions of the phases in question reflect the equilibrium compositions at 700 δC. Empirical equations are presented that accurately describe the temperature dependences ofa _γ,a _γ′, and δ over the range of temperatures of this investigation.     

Deformation in Ti3Al fatigued at room and elevated temperatures

The fatigue behavior of Ti₃Al in the temperature range 23 to 900 °C was investigated. With increasing temperature the fracture mode changed from transgranular cleavage to cleavage mixed with increasing intergranular failure. At the highest temperature microcracks parallel to the tensile axis were observed, accompanied by decreased planarity of the fracture surface. Little evidence of cyclic crack propagation was observed. The dislocation substructure was found to consist predominantly of planar tangles of dislocation in edge orientations at temperatures of 500 °C and above. No appreciable activity of dislocations having other Burgers vectors was observed. As a consequence of the limited number of active slip systems, fracture was controlled by slip incompatability at grain boundaries.